n, t = map(int, input().split())
a = list(map(int, input().split()))
itm = [0] * n
input()
for x in map(int, input().split()):
    itm[x - 1] = 1
now = 0
ans = 0
for i in range(n - 1):
    # 10*use+t > ti
    # use>(ti-t)/10
    now += itm[i]
    use = max(0, (a[i] - t) // 10 + 1)
    if use > now:
        ans = -1
        break
    ans += use
    now -= use
    t += 10 * use
    t -= a[i]
print(ans)